Maximum Number of points on a line from origin

app.js

/* line {x,y,dx,dy} */
var points = [{x:1,y:1},{x:2,y:2},{x:1,y:2},{x:1,y:3},{x:1,y:4}];


var maxNumberOfPoints = 1;


points.forEach(function(pointA) {
  var numberOfPointsA = 1;
  var atan = Math.atan2(pointA.y, pointA.x);
  points.forEach(function(pointB) {
     if(pointA == pointB) {
        return;
     }
     var atanJ = Math.atan2(pointB.y, pointB.x);
     if(atanJ == atan || Math.abs(atanJ) + Math.abs(atan) == Math.PI)  {
        //we got one!
        numberOfPointsA++;
     }
  });
  maxNumberOfPoints = Math.max(numberOfPointsA, maxNumberOfPoints);
});

console.log("maxNumberOfPoints atan2:", maxNumberOfPoints);

Thanks for the help @nfreymuth-telepathy

I guess my brain wasn't firing on all cylinders on Friday. I've changed your code a bit to account for more test cases, instead of tracking only the last angle I needed to keep an object (hash/map) to track the counts at each angle, this now works for all of my test cases:

/*** poor man's test data, all test cases should return 5 ***/
//vertical line test
//var points = [{x:1,y:1},{x:2,y:2},{x:1,y:2},{x:1,y:3},{x:-1,y:4},{x:10,y:0},{x:1,y:-1},{x:1,y:-2}];
//horizontal line test
//var points = [{x:2,y:2},{x:-15,y:2},{x:1,y:2},{x:1,y:1},{x:2,y:2},{x:2000,y:2},{x:1,y:-1},{x:1,y:-2}];
//diag line test, positive x, negative y slope
//var points = [{x:-1,y:3},{x:1,y:-1},{x:0,y:1},{x:-2,y:5},{x:0,y:0},{x:1,y:1},{x:2,y:2},{x:1,y:2},{x:-3,y:7}];
//diag line test, positive x, positive y slope, interspersed points
var points = [{x:-1,y:-3},{x:1,y:1},{x:0,y:-1},{x:-2,y:-5},{x:0,y:0},{x:1,y:1},{x:2,y:2},{x:1,y:2}];

var maxNumberOfPoints = 1;

points.forEach(function(pointA) {
  var angleCounts = {};//indexed by angle, value is number of points at that angle
  points.forEach(function(pointB) {
      if(pointA == pointB) {
        return;
      }
      var dx = pointB.x - pointA.x;
      var dy = pointB.y - pointA.y;
      var angleAB = Math.abs(Math.atan2(dy, dx));
      if(!(angleAB in angleCounts)) {
        angleCounts[angleAB] = 2;
      } else {
        angleCounts[angleAB]++;
      }
      console.log(angleCounts[angleAB], pointA, pointB, dx, dy, angleAB);
  });
  for(var a in angleCounts) {
    maxNumberOfPoints = Math.max(angleCounts[a], maxNumberOfPoints);
  }
});

console.log("maxNumberOfPoints:", maxNumberOfPoints);

I'm not sure what should be returned in the case there are duplicate points, right now it doesn't de-duplicate so it counts dupes towards the max.

Obviously if I was doing this for real I would use a real unit test suite like mocha or tape, but my poor mans un-comment method works for this :)